The present invention relates to a hydraulic system for a press comprising a press ram, at least one main press cylinder with a primary side for producing press thrust and a device for returning the press ram.
Hydraulic systems for a press, such as a bale press, presently comprise pumps, valves, at least one cylinder, a press ram and a tank for the hydraulic medium, which is usually oil. The different parts are connected by hoses and pipes to bring about the desired function.
The tank is usually located above a main press cylinder, which in turn is attached to the press ram. To the press ram is also attached a device for returning the press ram. The returning device can, for example, be at least one separate return cylinder alternatively located in the main press cylinder, which in this manner is double-acting.
The oil thus flows from the tank through at least one valve, for rapid filling, into the primary side of the main press cylinder, while the press ram is lowered to the object to be pressed, for example a pulp bale.
The press ram produces a gradually increasing pressure against the object. In order to create a final press thrust, oil is pumped from the tank into the primary side of the main press cylinder.
When the press ram returns, the oil is pressed from the primary side of the main press cylinder back to the tank.
When the oil is pressurized, air can be dissolved in the oil. When the pressure decreases, the air is thus released. The air in the oil has a negative effect on the hydraulic system. In order to provide for the air to possibly diffuse out of the oil, there is a large volume of oil in the tank. The oil volume in the tank is usually dimensioned to three times the pump flow per minute. The large oil volume should thus be sufficient to provide the oil with such a stay-time and a surface exposed to the atmosphere, such that air can diffuse out of the oil.
Another object of the tank is to take up variations in the oil level during movements of the press ram.
In order to obtain a pressure balance in the tank at rapid level variations, the tank is connected to the atmosphere through a filter. Large air volumes pass through the filters. The filter is close-meshed, in order to prevent particles from the air from following along into the tank, and thus contaminate the oil. The filter must thus have a high flow capacity, because pressure variations in the tank shall not be permitted when the oil level rapidly changes.
The present state of art has a problem in that hydrolysis causes decomposition of seals therein. The air passing through the air filter carries with it moisture which is absorbed by the oil. This moisture causes hydrolysis damage to the seals.
Although the tank is large and contains a considerable amount of oil, there is always a certain amount of air in the oil. Therefore, cavitation problems can arise, for example in pumps and cylinders.
Another problem is, that the oil is disintegrated by oxidation. The oxygen oxidizing the oil originates both from the moisture and from the air in the oil.
The air filter as such also causes problems. Since it must be close-meshed, it becomes readily clogged.
The large tank, which must be located above the main press cylinder so that the oil can flow into the primary side thereof, implies an elevated structure.
In accordance with the present invention, these and other problems have now been solved by the invention of a hydraulic system for operating a press comprising a press ram, a press cylinder including a primary side for activating the press ram and press ram return means for returning the press ram, and at least one flow amplifier comprising a piston including a primary side and a secondary side, the primary side of the piston having a large active piston area and the secondary side of the piston having a small active piston area, the primary side of the piston being connected to the primary side of the press cylinder and the secondary side of the piston being connected to the press ram return means. A portion of the hydraulic system comprising the connection between the primary side of the piston and the primary side of the press cylinder is pressurized. Preferably, substantially the entire hydraulic system is pressurized.
In accordance with one embodiment of the hydraulic system of the present invention, the press ram return means includes a first total stroke volume and the secondary side of the piston includes a second total stroke volume, the first total stroke volume being greater than the second total stroke volume.
In accordance with another embodiment of the hydraulic system of the present invention, the system includes a pump connected to the secondary side of the piston for maintaining pressure therein.
In accordance with another embodiment of the hydraulic system of the presnt invention, the system includes at least one trap for separating water and air from a hydraulic medium used in the hydraulic system.
According to the present invention, the above problems are solved by providing a flow amplifier operating in counterphase to the main press cylinder in the hydraulic system.
In view of the provision of such a flow amplifier the tank, which according to known techniques is located above the main press cylinder, can now be eliminated.
During lowering of the press ram, the oil is pressed out of the device for returning the press ram, such as the return cylinders, to the flow amplifier, which presses the oil into the primary side of the main press cylinder. The dead weight of the press ram can thus be utilized for amplifying filling on the secondary side of the main press cylinder, which is not possible in accordance with the known state of art.
By use of such a flow amplifier, rapid variations in the oil level, for example in a tank, are avoided. These rapid level variations can cause moisture, air and particles from the air to get into the oil.
At least the part of the hydraulic system between the flow amplifier and the primary side of the main press cylinder is pressurized. Since the supply of the oil to the primary side of the main press cylinder thereby takes place under pressure, a higher flow rate and smaller valve size can now be achieved as compared to that in the prior art.
According to one embodiment of the present invention, substantially the entire hydraulic system is pressurized. Necessary pumps are also supplied from the closed part of the hydraulic system. As a result, only a small tank is required to compensate for heat expansion and oil leakage. The total amount of oil in the system can now also be reduced to, for example, about one-third of the oil volume required in known systems.
Furthermore, if the pumps are fed from the closed part of the hydraulic system, the oil can be de-aired and de-humidified, while substantially no new air and moisture is supplied to the oil.